1. Field of the Invention
The present invention relates to a radio communication system, a receiver, and a communication method.
2. Description of Related Art
A radio communication system for use with a vehicle is known that performs information processing by carrying out radio communication between an in-vehicle electronic device and a portable device or a detection device.
The radio communication system includes a remote keyless entry (RKE) system. In the RKE system, the user of a vehicle operates a portable device to send a radio wave, which includes unique information, from the portable device to the vehicle. The in-vehicle electronic device authenticates the portable device, based on the radio wave received from the portable device, to allow the user to lock or unlock the door.
The radio communication system also includes a tire pressure monitoring system (TPMS). The TPMS uses a detection device, provided in a vehicle, to detect the tire pressure. The detection device wirelessly sends the information, which indicates the tire pressure, to the in-vehicle electronic device. The in-vehicle electronic device monitors the tire pressure based on the radio wave received from the detection device.
A technology for use with the in-vehicle electronic device is known that allows one receiver to receive multiple types of transmission radio waves that are in the same frequency band and uses the same modulation method but that differ in the frequency of the baseband signals (for example, see Japanese Patent Application Publication No. 2005-236556 (JP 2005-236556 A)).
The receiver is configured to receive a radio wave wile switching between the three types of radio waves: radio wave for remote keyless entry system received from the portable device, radio wave for smart keyless entry system received from the portable device, and radio wave sent from the detection device provided on each wheel for detecting a tire pressure. The three types of radio waves are in the same frequency band and use the same modulation method but differ in the frequency of the baseband signals. In the receiver, the baseband signal, obtained by demodulating the radio wave signal received via the antenna, is passed from the detector circuit to the low-pass filter (LPF) and, after being shaped, output as the demodulated signal. The cutoff frequency fc of the LPF is changed to one of the three values, suitable for receiving one of the above three types of radio waves, according to the filter switching signal.
While a vehicle is parked during which the ignition switch of the vehicle is off and all doors are closed, the receiver described above receives the radio wave at regular intervals. More specifically, at each predetermined reception processing time, the receiver changes the filter switching signal to receive one of the three types of radio waves described above. The filter switching signal is changed so that the cutoff frequency fc for receiving each radio wave is set to receive the radio waves cyclically in the following order: the radio wave for the remote keyless entry system, the radio wave for the tire pressure monitoring system, the radio wave for remote keyless entry system, and the radio wave for the smart keyless entry system.
The receiver receives the signal at regular intervals also while the vehicle is traveling. The receiver is configured in such a way that, while the vehicle is traveling, the filter switching signal is not changed but only the radio wave for the tire pressure monitoring system is received.
The problem with the configuration described above is that the receiver receives only the radio wave for the tire pressure monitoring system while the vehicle is traveling and, therefore, cannot receive the radio wave for the remote keyless entry system that requires the ignition switch be on. More specifically, the remote keyless entry system cannot accept registration of a key, or performs a malfunction diagnosis of the receiver.
On the other hand, if the receiver is configured to receive the radio wave for the remote keyless entry system while the vehicle is traveling, the vehicle keeps traveling with the receiver capable of receiving the radio wave for the remote keyless entry system. In such a case, the user sometimes cannot respond properly to a particular situation such as a tire puncture. More specifically, the tire pressure monitoring system does not work timely and therefore cannot issue an adequate warning.